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Let's say you have a stick thats one lightyear long...

Zeze

Zeze

Lifer
From one end of the stick, you tilt its orientation few degrees. Will the other end of the stick, one lightyear away from you, move instantaneously?
 
IronWing said:
No. Even with short sticks the response is delayed as the shear force propagates up the stick. The stick deflects while the force is applied and propagates.
Click to expand...
What is the speed of the force propogating? How do we calculate this?
 
Zeze said:
What is the speed of the force propogating? How do we calculate this?
Click to expand...
It is material dependent, based on the molecular structure of the material, the nature of inter-molecular bonding, and any macroscopic structure in the material.
 
Zeze said:
What is the speed of the force propogating? How do we calculate this?
Click to expand...
IronWing said:
It is material dependent, based on the molecular structure of the material, the nature of inter-molecular bonding, and any macroscopic structure in the material.
Click to expand...
But the speed is less than the speed of sound in the material. Here's a way to estimate the speed at which the wave propagates to the end of the stick:

en.wikipedia.org

S wave - Wikipedia

en.wikipedia.org en.wikipedia.org
 
What if it's a laser? As you change the angle of the laser, does the point-of-light on the object the laser lands on move faster than the speed of light?
(I believe the answer is yes it does, because the movement of that bright spot itself does not involve the movement of information..it's only useable information that can't exceed the speed of light, as I understand/remember it...something that doesn't contain accessible information can in fact exceed that speed.)

I might be wrong...maybe it works the same as the stick?

What about the point where two non-parallel sticks intersect? As you tilt one of them does that point (which is not itself a physical object) move faster than light?
 
At the fastest it will travel speed of light. Which is kinda mind boggling when you think about it. If you have a long rod made from a theoretical material that has zero stretch or bend etc and is 100% solid, the propagation is still limited to speed of light and is not instant.

With any other material it will technically be slower due to the material properties and how it will compress/stretch and it will essentially form a wave going all the way to the end, or it may even die before it gets there.

As for a laser, think of it as if you are spraying something with the hose, if you move it very fast, the water that was already coming out is still going to go in the same direction you were pointing.

At least I think that's how it works...
 
pmv said:
What if it's a laser? As you change the angle of the laser, does the point-of-light on the object the laser lands on move faster than the speed of light?
(I believe the answer is yes it does, because the movement of that bright spot itself does not involve the movement of information..it's only useable information that can't exceed the speed of light, as I understand/remember it...something that doesn't contain accessible information can in fact exceed that speed.)
Click to expand...
The light that is hitting the spot one light year away when you decide to move the laser left your laser a year ago. Those photons wil be absorbed or reflected. As you move the laser, photons leaving the laser are set on their courses fanning out perdendicular to the arc of your movement. When you stop moving the laser, those photons leaving the laser will eventually hit a new spot a year from now (assuming there is something out there to hit).

Edit: If you are seeing a bright spot where your laser hit an object out there, that reflected light is now two light years old. If you move the laser, you will have to wait two years to see the movement of the bright spot.
 
IronWing said:
The light that is hitting the spot one light year away when you decide to move the laser left your laser a year ago. Those photons wil be absorbed or reflected. As you move the laser, photons leaving the laser are set on their courses fanning out perdendicular to the arc of your movement. When you stop moving the laser, those photons leaving the laser will eventually hit a new spot a year from now (assuming there is something out there to hit).

Edit: If you are seeing a bright spot where your laser hit an object out there, that reflected light is now two light years old. If you move the laser, you will have to wait two years to see the movement of the bright spot.
Click to expand...

You're right, though I garbled the memory of the question. The question should have been, does the light spot move faster than light from the point-of-view of someone standing on the surface of the planet (or whatever) that you are shining the light on? I do think the answer is yes, because it's not really a 'thing' moving, it doesn't transport information from the original location of the light spot to the new location. The speed limit of c is all about information content (which is how some of those attempts to reconcile QM and relativity get round the contradiction between the two, if I recall, from a very very long time ago).

I cheated and googled it, and this is in fact what I was thinking of - not exactly the scenario I described, because I garbled the memory..

physics.stackexchange.com

Is it really possible to break the speed of light by flicking your wrist with a laser pointer?

Minutephysics has a popular YouTube video called "How to break the speed of light". In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of
physics.stackexchange.com physics.stackexchange.com
 
I'm with you now. Yeah, if you swing the laser around 180 degrees, the "bright spot" travels across the universe in the time it takes to flick your wrist.
 
For a brief moment I was curious why the laser pointer example worked differently to the stick, and whether it in fact worked differently or not....but really it's not that surprising, I realise, as the light beam is not an object the way the stick is. (Though how do star-wars light-sabres work then, eh? A magnetic bottle of plasma? So a light-year long light sabre would behave like a stick not a laser beam, I guess?)
 
This video explains some portion of OP's question.


The light sabres only exist in science fictions, at least for now.
 
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pmv said:
What if it's a laser? As you change the angle of the laser, does the point-of-light on the object the laser lands on move faster than the speed of light?
(I believe the answer is yes it does, because the movement of that bright spot itself does not involve the movement of information..it's only useable information that can't exceed the speed of light, as I understand/remember it...something that doesn't contain accessible information can in fact exceed that speed.)

I might be wrong...maybe it works the same as the stick?

What about the point where two non-parallel sticks intersect? As you tilt one of them does that point (which is not itself a physical object) move faster than light?
Click to expand...
Laser is easy, it's speed of light.

My original Q is very curious because it's a static object.
 
Even if you have a laser beam that's one light year long that hits somewhere in the universe contentiously, even if you starts to move the laser beam at an angle, it will take one year for the photon to arrive at that spot, all the photons that already left the laser device earlier will continue to hit the same spot.

The one standing on that spot will not see any difference for a full one year.
 
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mxnerd said:
Even if you have a laser beam that's one light year long that hits somewhere in the universe contentiously, even if you starts to move the laser beam at an angle, it will take one year for the photon to arrive at that spot, all the photons that already left the laser device earlier will continue to hit the same spot.

The one standing on that spot will not see any difference for a full one year.
Click to expand...

I don't think that's the point though. Those standing on the spot will experience the spot moving at a speed higher than 'c'. I think. I find it more confusing the more I think about it.

I think it's more-or-less the same thing as this

en.wikipedia.org

Superluminal motion - Wikipedia

en.wikipedia.org en.wikipedia.org
 
pmv said:
I don't think that's the point though. Those standing on the spot will experience the spot moving at a speed higher than 'c'. I think. I find it more confusing the more I think about it.

The video you mentioned earlier

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What this guy's who owns minutephysics channel says is that when you flip you wrist in your backyard, it takes only 0.0005 seconds to across the moon's surface so the speed is 20 times of speed of light.

Sun's diameter is 400 times of the moon, but the same size as moon in human's eyes.

So if I flip my wrist with a laser beam across sun's surface, it will be 8000 times of speed of light?

I would ask a NASA scientist before trusting a youtuber.
 
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pmv said:
For a brief moment I was curious why the laser pointer example worked differently to the stick, and whether it in fact worked differently or not....but really it's not that surprising, I realise, as the light beam is not an object the way the stick is. (Though how do star-wars light-sabres work then, eh? A magnetic bottle of plasma? So a light-year long light sabre would behave like a stick not a laser beam, I guess?)
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Basically magic.
 
mxnerd said:
What this guy's who owns minutephysics channel says is that when you flip you wrist in your backyard, it takes only 0.0005 seconds to across the moon's surface so the speed is 20 times of speed of light.

Sun's diameter is 400 times of the moon. So if I flip my wrist with a laser beam across sun's surface, it will be 8000 times of speed of light?

I would ask a NASA scientist before trusting a youtuber.
Click to expand...
The dot will appear to move that fast across the surface of the sun but none of the reflected light making up the dot is moving any faster than c.
 
No. Even if the movement of the stick from beginning to end (tip to tip) moves at the speed of light, it will still take a year.
 
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mxnerd said:
What this guy's who owns minutephysics channel says is that when you flip you wrist in your backyard, it takes only 0.0005 seconds to across the moon's surface so the speed is 20 times of speed of light.

Sun's diameter is 400 times of the moon, but the same size as moon in human's eyes.

So if I flip my wrist with a laser beam across sun's surface, it will be 8000 times of speed of light?

I would ask a NASA scientist before trusting a youtuber.
Click to expand...
A better way to explain this: The dot doesn't move at all. The chunk of moon the laser hit at the beginning stops glowing when the laser moves off it. The chunk next to it glows instead as the laser moves across it, then the next chunk, and the next. The change in location of the dot is not limited by the speed of light as the dot isn't a thing that moves.
 
IronWing said:
A better way to explain this: The dot doesn't move at all. The chunk of moon the laser hit at the beginning stops glowing when the laser moves off it. The chunk next to it glows instead as the laser moves across it, then the next chunk, and the next. The change in location of the dot is not limited by the speed of light as the dot isn't a thing that moves.
Click to expand...
Thanks! That really explains it. The video saying it's breaking speed of light is completely misleading.
 
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